Never buy anything from Amazon without checking eBay first. I think every generic thing in the world is available on eBay, IME at about half the price.
(May not be worth it for books and other media. But I just bought a pile of stuff for moving house with, down to replacement light bulbs for when we take our expensive daylight CFLs with us. And it's always fun to just casually buy six rolls of packing tape and a 100-metre roll of bubble wrap, even when you have an actual reason to.)
For books, use bigwords.com; it searches a bunch of book sellers and finds the cheapest price for the book(s) you're looking for. I got most of my college textbooks that way.
Thus spake Eliezer:
It seems that many here might have outlandish ideas for ways of improving our lives. For instance, a recent post advocated installing really bright lights as a way to boost alertness and productivity. We should not adopt such hacks into our dogma until we're pretty sure they work; however, one way of knowing whether a crazy idea works is to try implementing it, and you may have more ideas than you're planning to implement.
So: please post all such lifehack ideas! Even if you haven't tried them, even if they seem unlikely to work. Post them separately, unless some other way would be more appropriate. If you've tried some idea and it hasn't worked, it would be useful to post that too.